Connected: An Internet Encyclopedia
RFC 1812 - 1.1.2 Requirements

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### 1.1.2 Requirements

In this memo, the words that are used to define the significance of each particular requirement are capitalized. These words are:

MUST
  This word means that the item is an absolute requirement of the specification. Violation of such a requirement is a fundamental error; there is no case where it is justified.

MUST IMPLEMENT
  This phrase means that this specification requires that the item be implemented, but does not require that it be enabled by default.

MUST NOT
  This phrase means that the item is an absolute prohibition of the specification.

SHOULD
  This word means that there may exist valid reasons in particular circumstances to ignore this item, but the full implications should be understood and the case carefully weighed before choosing a different course.

SHOULD IMPLEMENT
  This phrase is similar in meaning to SHOULD, but is used when we recommend that a particular feature be provided but does not necessarily recommend that it be enabled by default.

SHOULD NOT
  This phrase means that there may exist valid reasons in particular circumstances when the described behavior is acceptable or even useful. Even so, the full implications should be understood and the case carefully weighed before implementing any behavior described with this label.

MAY
  This word means that this item is truly optional. One vendor may choose to include the item because a particular marketplace requires it or because it enhances the product, for example; another vendor may omit the same item.

Next: 1.1.3 Compliance

Connected: An Internet Encyclopedia
RFC 1812 - 1.1.2 Requirements